Multi-speed automatic vehicle transmissions are well-known. In such a transmission a plurality of speed ratios are independently selectable under the command of a control system reliant upon, for example, vehicle speed and accelerator pedal position. In order to improve fuel economy the number of forward speed ratios may exceed eight. A large number of speed ratios also allows for an appropriate speed ratio in the event of a significant change in vehicle mass due to loading thereof, and for towing.
In benign circumstances, for example on a level road in good weather, an automatic transmission may start from rest in a speed ratio which is not the lowest speed ratio. Starting from rest in second or third speed ratio may be well within vehicle capabilities, and may improve the launch feel of the vehicle by reducing the number of speed ratio changes as the vehicle approaches highway speed. Such a technique may also improve vehicle fuel economy.
In for example a torque converter automatic transmission with eight forward speeds, launching from second or third speed ratio is more comfortable because there is a lower gain between throttle response and the delivery of torque to the driving wheels. In other words greater movement of the accelerator pedal is provided for a given response of the engine, and sudden pedal movements are masked. Also roll-out downshifts (zero accelerator pedal position) may be smoother if the lowest speed ratios are avoided—thus the shift shock of a downshift from second to first speed ratio may be obviated.
It is also known to provide special transmission shift maps in adverse weather or ground conditions. For example in ice and snow a transmission may start from rest in a higher speed ratio to minimize the risk of wheel spin, and the same technique may be used in muddy or wet conditions, for example when travelling off-road.
The appropriate shift map may be selected automatically without intervention of the driver; for example an external temperature sensor may lock out the lowest forward speed ratio(s) if air temperature drops below 0° C.
Systems of selecting one of several speed ratios for launch are generally insensitive to road gradient or obstacles such as kerbs or rocks, and in consequence starting from rest may be problematic if the transmission control system selects a shift map which provides for launch in a higher speed ratio. Typically, in such circumstances, the vehicle driver presses the accelerator pedal further, and eventually the transmission control system recognizes that a shift map with a lower launch speed ratio is appropriate. The transmission downshifts at a relatively high engine torque, which may be uncomfortable for the vehicle occupants.
It would be desirable to provide for selection of an appropriate launch ratio in a manner which is not noticeable to the vehicle driver, so that a normal range of accelerator pedal movement is better maintained.